In recent years, the application range of magnetic recording apparatuses such as magnetic disk apparatuses, flexible disk apparatuses, and magnetic tape apparatuses has increased remarkably and their importance has also increased. Therefore, a technique has been developed for significantly improving the recording density of a magnetic recording medium used for these apparatuses. In particular, the introduction of an MR (magneto resistive) head and a PRML (partial response maximum likelihood) technique has accelerated improvement in surface recording density of the magnetic recording medium. In recent years, with the introduction of GMR (giant magnetoresistance) heads and TuMR (tunneling magnetoresistance) heads, the increase in the surface recording density of the magnetic recording medium has increased significantly and continues to increase at a pace of 30% to 40% each year.
In this way, there is a need for an increase in recording density of magnetic recording medium and there is also a need for an increase in coercive force, a high signal-to-noise ratio (SNR), and high resolution of a magnetic recording layer. In a longitudinal magnetic recording system which has generally been used, with the increase in linear recording density, recording magnetic domains adjacent to a magnetization transition region mutually weaken their magnetizations, which is called self-demagnetization. In order to prevent the self-demagnetization, it is necessary to reduce the thickness of the magnetic recording layer to increase the shape magnetic anisotropy.
On the other hand, when the thickness of the magnetic recording layer is reduced, the strength of an energy barrier for maintaining the magnetic domain is substantially equal to that of thermal energy, and the phenomenon (heat fluctuation phenomenon) in which an amount of recorded magnetization is reduced due to a temperature variation is not negligible. Accordingly, it is said that the heat fluctuation phenomenon determines the limit of the linear recording density.
In recent years, a medium having an AFC (anti-ferromagnetic coupling) structure has been proposed as the technique for improving the linear recording density in the longitudinal magnetic recording system, thereby solving thermo-magnetism relaxation which is a problem in the longitudinal magnetic recording system.
As a technique for further improving the surface recording density, a perpendicular magnetic recording technique has attracted attention. A medium is magnetized in the in-plane direction in the longitudinal magnetic recording system, but a medium is magnetized in a direction substantially perpendicular to the surface of the medium in the perpendicular magnetic recording system. Accordingly, since it is possible to avoid the self-demagnetization that prevents the increase in linear recording density in the longitudinal magnetic recording system, it is considered that the perpendicular magnetic recording system is suitable for high-density recording. In addition, since the perpendicular magnetic recording system can keep the thickness of the magnetic layer constant, it is possible to relatively reduce the influence of the thermo-magnetism relaxation which causes a problem in the longitudinal magnetic recording system.
In general, a perpendicular magnetic recording medium is obtained by sequentially forming a seed layer, an intermediate layer, a magnetic recording layer, and a protective layer on a nonmagnetic substrate. In many cases, a lubricant layer is formed on the surface of the protective layer. In many cases, a magnetic film which is called a soft magnetic underlying layer is disposed below the seed layer. The intermediate layer is formed in order to further improve the characteristics of the magnetic recording layer. In addition, the seed layer serves to align the crystal particles of the intermediate layer and the magnetic recording layer, and to control the shape of magnetic crystals.
The improvement in crystalline orientation and the decrease in crystal grain size of the magnetic recording layer are important in the manufacturing of a perpendicular magnetic recording medium with good characteristics. That is, in the perpendicular magnetic recording medium, the magnetic recording layer is formed of a Co alloy material and the crystal structure thereof is a hexagonal closest-packed (hcp) structure. It is important that a (002) crystal face of the hexagonal closest-packed structure is parallel to the surface of the substrate, that is, a crystal c-axis ([002] axis) is aligned in a direction substantially perpendicular to the surface of the substrate with the least possible disorder.
In order to align the crystal particles of the magnetic recording layer with the least possible disorder, the intermediate layer of the perpendicular magnetic recording medium has been made of Ru having the same hexagonal closest-packed structure as the magnetic recording layer. Since the crystal particles of the magnetic recording layer epitaxially grow on the (002) crystal face of Ru, a magnetic recording medium with good crystalline orientation is obtained (for example, see Patent Citation 1).
That is, since the orientation of the magnetic recording layer is improved by enhancing the degree of (002) crystal face orientation of the Ru intermediate layer, the (002) crystal face orientation of Ru should be improved to improve the recording density of the perpendicular magnetic recording medium. However, when an Ru film is formed directly on an amorphous underlying layer, the thickness of the Ru film increases to obtain the good crystalline orientation and the amorphous Ru weakens the attraction of magnetic flux from a head in the underlying layer which is formed of a soft magnetic material at the time of recording. Therefore, in the past, a seed layer for orientation on the (111) crystal face of a face-centered cubic structure is interposed between the underlying layer and the Ru intermediate layer (For example, see Patent Citation 2). Since the seed layer having the face-centered cubic structure can provide a high crystalline orientation property even with a thickness of about 5 nm, Ru on the seed layer having the face-centered cubic structure can provide a high crystalline orientation property with a thickness smaller than that of Ru formed directly on the underlying layer.
However, in order to further improve the recording density, it is necessary to further strengthen the attraction of magnetic flux from the head in the underlying layer. When a material having a high saturated magnetization Bs is selected as a soft magnetic material to strengthen the attraction of magnetic flux, a part of the soft magnetic material is finely crystallized and thus the unevenness of the surface of the underlying layer increases, thereby deteriorating the orientation of the magnetic recording layer. As another technique for strengthening the attraction of magnetic flux, the thickness of the nonmagnetic layers from the intermediate layer below the magnetic recording layer to the seed layer can be reduced. However, when the seed layer having the face-centered cubic structure used in the related art has a thickness equal to or smaller than 5 nm, the orientation of the intermediate layer is rapidly deteriorated and thus the thickness cannot be set to be equal to or smaller than 5 nm. In view of the above-mentioned situation, in order to further improve the recording density, there is a need for a seed layer which can maintain the orientation of the intermediate layer or the magnetic recording layer to a certain extent even with a thickness of 5 nm or less.
A decrease in crystal grain size of the magnetic recording layer is necessary for improving the recording density. In the existing crystalline seed layer as well as the seed layer having the face-centered cubic structure, since a single Ru crystal grain of the intermediate layer epitaxially grows on a single crystal grain of the seed layer, the decrease in crystal grain size of the seed layer can be considered as a method of reducing the crystal grain size of Ru of the magnetic recording layer or the intermediate layer. There have been various studies on the materials of the seed layer and the intermediate layer and the film forming method, but it was not possible to reduce the crystal grain size of the seed layer while maintaining the crystalline orientation of the intermediate layer or the magnetic recording layer.
As another method of reducing the crystal grain size of the intermediate layer or the magnetic recording layer on the seed layer, it was reported that the intermediate layer can be formed with a granular structure including a crystal grain portion of Ru and a grain boundary portion of oxide and the like surrounding the crystal grain portion, which is generally used in the magnetic recording layer (for example, Non-Patent Citation 1). When the grain boundary portion of the intermediate portions is thickened by increasing the amount of oxide using this method, it is possible to reduce the crystal grain size by as much. When an oxide magnetic layer such as a CoCrPt—SiO2 layer is formed on the intermediate layer, the granular structures are connected to each other from the intermediate layer to the magnetic recording layer, the decrease in grain size of the magnetic crystal grains or the segregation of oxide is promoted to reduce the noise, and the improvement of the recording and reproducing characteristics is thus expected.
However, since the change of the intermediate layer to the granular structure is not the decrease in grain size of the seed layer, the number of crystal grains per unit area is not changed. Accordingly, in further increasing the recording density, the number of magnetic crystal grains per bit is decreased with the decrease in area per bit, thereby causing a decrease in signal intensity. Due to the increase in the ratio of oxide grain boundaries, Co in the magnetic crystal grains is oxidized, thereby causing a further decrease in signal intensity. In order to improve the recording and reproducing characteristics to cope with the increase in recording density, it is necessary not only to simply decrease the grain size of the magnetic crystal, but also to increase the density of the magnetic crystal grains.
Patent Citation 3 discloses that by constructing the orientation control layer out of a hcp-structured or fcc-structured layer and a hcp-structured layer from the substrate and setting the Ru contact angle therebetween to a range of 50 to 120 degrees, it is possible to manufacture a perpendicular magnetic recording medium with a high recording density in which the c axis of the crystal structure, that is, the hexagonal closest-packed structure, of the perpendicular magnetic layer is aligned with a very small angle variance with respect to the surface of the substrate and the average particle size of the crystal grains of the perpendicular magnetic layer is very small. However, in order to improve the recording and reproducing characteristics in the future, it is necessary to provide a perpendicular magnetic recording medium while maintaining excellent recording and reproducing characteristics in which the decrease in grain size of the magnetic crystal grains is consistent with the increase in recording density and the perpendicular orientation property of the magnetic recording layer. There is a need for a perpendicular magnetic recording medium which can be easily manufactured without causing the above-mentioned problems.    [Patent Citation 1] JP-A-2001-6158    [Patent Citation 2] JP-A-2005-190517    [Patent Citation 3] JP-A-2006-155865    [Non-Patent Citation 1] APPLIED PHYSICS LETTERS, vol. 89, pp. 162504
The invention is made in view of the above-mentioned situation, and an object of the invention is to provide a magnetic recording medium which can record and reproduce high-density information by causing a decrease in grain size of magnetic crystal grains to be consistent with an increase in recording density while maintaining perpendicular orientation of a magnetic recording layer, a manufacturing method thereof, and a magnetic recording and reproducing apparatus employing the magnetic recording medium.